After pneumonectomy, compensatory growth occurs in the remaining lung. The vascular response during this growth and how individual lobes of the lung respond are not well understood. The aim of our study was to characterize vascular growth among individual lobes of the lung after pneumonectomy and determine whether changes in relative blood flow correlate with growth.Rats underwent left pneumonectomy, and lobe weights and volumes of the right lung were measured 21 days later. Arterial growth was quantitated from arteriograms of each lobe after barium perfusion. Changes in relative blood flow were assessed by using radiolabeled microspheres. Expression of proliferating cell nuclear antigen was measured by means of Western blot analysis.After pneumonectomy, weight and volume indices of all lobes were significantly increased compared with those seen in sham control animals. Arterial growth occurred in all lobes after pneumonectomy, with the greatest increases occurring in the upper and middle lobes. In addition, a differential distribution of blood flow was observed where the upper and middle lobes contained the highest degree of relative flow. Pneumonectomy produced hyperplasic growth in all lobes, as indicated by significantly increased proliferating cell nuclear antigen expression. Proliferating cell nuclear antigen expression correlated with arterial growth in that increased and prolonged expression occurred in the upper lobe.These results show that left pneumonectomy induces significant, nonuniform, compensatory growth in all lobes of the right lung. Arterial growth occurred in each lobe after pneumonectomy, but preferentially higher vascular growth and cell proliferation in the upper lobe positively correlated with higher relative blood flow in this lobe.